1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices and particularly to a SAW filter applied to digital CATV (community antenna television, cable television) system.
2. Description of the Background Art
A SAW filter of transversal type has been known as one of SAW devices. The transversal SAW filter has a property that a desired characteristic can readily be accomplished by means of various weighting methods.
FIG. 6 shows a conventional SAW filter 1. Referring to FIG. 6, SAW filter 1 includes a piezoelectric substrate 2, an input IDT (interdigital transducer) 3, an output IDT 4, a shield electrode 5, and an adsorbing material 6. Input IDT 3 and output IDT 4 are constructed of interdigital electrodes.
The interdigital electrodes exhibit a basic property of generating surface acoustic waves as well as properties as electric capacitance. The electric capacitance is naturally generated because electrodes are arranged opposite each other with a short distance therebetween. It is thus impossible to control the capacitance independently of generation of surface acoustic waves. With a value of the naturally generated electric capacitance represented by CT, the imaginary component of an input impedance of the electrode discussed above is expressed by 1/(jxcfx89xc2x7CT). When there is a great difference in this value of the imaginary component between input and output, the input and output accordingly have unbalanced impedances and a resultant filter characteristic exhibits a slope.
When SAW filter 1 shown in FIG. 6 is applied to an intermediate frequency (IF) for digital broadcasting, an extremely steep characteristic is required. Then, adjustment is difficult of a subtle impedance balance between input and output by means of only the weighting of interdigital electrodes.
Consequently, an impedance imbalance occurs between input and output that causes a slope of an inband characteristic as shown in FIG. 7, and thus insertion loss and inband ripple are deteriorated. The reason is that employment of a complex weighting function makes it impossible to cancel frequency characteristics of respective parasitic impedances of the input electrode (IDT) and the output electrode (IDT).
The present invention has been made to solve the problem above. One object of the invention is to eliminate the slope of the inband characteristic and accordingly improve the insertion loss and inband ripple by correcting an impedance imbalance between input and output electrodes.
A surface acoustic wave device according to the present invention includes a piezoelectric substrate, an input electrode (IDT) and an output electrode (IDT) formed on the piezoelectric substrate, and inductor means formed on the piezoelectric substrate for correcting an impedance imbalance between the input electrode and the output electrode.
The inductor means thus provided enables an inductance of a desired value to be added to the input electrode or the output electrode thereby changes an input impedance value. Specifically, when an element of self-inductance L is added in parallel to one of the electrodes, the imaginary component of an input impedance is defined by 1/{jxcfx89xc2x7CTxe2x88x92j/(xcfx89L)} and thus the input impedance value changes. In this way, inductor means having a proper self-inductance value can be provided so as to adjust the input impedance value to a desired value and accordingly correct an impedance imbalance between input and output electrodes.
A metallic pattern may be used as the inductor means. This metallic pattern is preferably a long metallic pattern of meander type or spiral type. The long metallic pattern has a great self-inductance so that the impedance imbalance between the input electrode and the output electrode can be corrected over a greater range.
Preferably, the inductor means is connected in parallel with the output electrode. Accordingly, the input impedance value can be controlled as discussed above and correction is possible of the impedance imbalance between the input and output electrodes.
The input and output electrodes are preferably interdigital electrodes, and a self-inductance value of the inductor means is determined to balance input impedance values of respective input and output electrodes.
The impedance imbalance between the input and output electrodes can thus be corrected.
The inductor means preferably has its self-inductance value from 10 nH to 100 nH. Further, the surface acoustic wave device of the invention is preferably applied to an intermediate frequency band of digital television system.
It has been known customarily that an electrode has an electric capacitance of approximately 1 pF in an intermediate frequency band (band of several tens of MHz) for images. In this case, an input impedance has its magnitude on the order of several ohms. For correcting the input impedance on the order of several ohms, an impedance should be added in parallel that is on the same order or greater by one order. The reason is that an impedance added thereto that is too great in magnitude substantially has no influence while an impedance added thereto that is too small in magnitude causes inversion of the sign of the impedance and accordingly the capacitive feature changes to an inductive feature. Regarding the intermediate frequency band, an impedance ranging from several ohms to several tens of ohms can be achieved with 10 nH to 100 nH. Accordingly, an impedance imbalance between input and output electrodes applied to the intermediate frequency band can be corrected by defining the self-inductance value of the inductor means as 10 nH to 100 nH.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.